The leading cause of lower back pain arises from rupture or degeneration of lumbar intervertebral discs. Pain in the lower extremities is caused by the compression of spinal nerve roots by a bulging disc, while lower back pain is caused by collapse of the disc and by the adverse effects of bearing weight through a damaged, unstable vertebral joint. One conventional method of managing these problems is to remove the problematic disc and replace it with a prosthetic disc that allows for the natural motion between the adjacent vertebrae (xe2x80x9ca motion discxe2x80x9d).
U.S. Pat. No. 5,676,701 (xe2x80x9cYuanxe2x80x9d) discloses a motion disc including a first component whose inner surface comprises a concave inner portion having a 360xc2x0 circumference and a convex peripheral portion, and an opposing second component whose inner surface comprises a conforming convex inner portion and a convex peripheral portion. The convex/concave contours of the opposing inner portions forms a ball-and-socket design that allows unrestricted pivotal motion of the device, while the opposing convex peripheral contours allow flexion/extension bending motion in the range of about 20-30xc2x0.
However, neither component of the Yuan device limits the extent to which the articulation surface may become vertical disengaged. Restraint of the pivotal motion in this device is provided by the contours of the opposing peripheral surfaces. Moreover, the articulating surfaces of Yuan are relatively small in area, limited to the middle third of the device.
U.S. Pat. No. 5,507,816 (xe2x80x9cBullivantxe2x80x9d) discloses a three-piece motion disc comprising an upper piece having a flat lower surface, a middle spacer having a flat upper surface and a convex lower surface, and a lower piece having a concave upper surface. The articulating convex and concave surfaces form an articulating interface that allows pivotal motion, while the flat surfaces form a translation interface that allows translational motion. Bullivant further teaches that the natural tension of the vertebrae ensures that the vertebrae are biased together to trap the spacer in place, and that the 90xc2x0 extension of the convex and concave surfaces virtually eliminates any chance of the spacer escaping from between the upper and lower pieces under normal pivotal movement of the vertebrae.
Since Bullivant relies upon natural tension to keep the components in place, none of the components is designed to limit the vertical disengagement of the articulation surfaces. Restraint of the pivotal motion appears to be provided by the peripheral portions of the inner surfaces of the upper and lower components (i.e., downwardly flat facing surface 28 and upwardly flat facing surface 30). Moreover, Bullivant does not teach a two piece design. The articulating surfaces of Bullivant are relatively small in area, limited to the middle half of the device.
U.S. Pat. No. 6,113,637 (xe2x80x9cGillxe2x80x9d) discloses a motion disc having a ball and socket articulation, wherein the trough of the socket has a flat portion. The ball and socket geometry provides pivotal motion while the flat portion of the trough allows the ball to slide, thereby providing some translation motion.
Gill relies upon independent rigid fixation of each component to its respective vertebral endplate as the means for limiting vertical disengagement of its articulation surfaces. Restraint on pivotal motion of the device is provided by rim 57 extending laterally from the perimeter of the socket, and by the rigid fixation of the upper and lower components to their respective vertebrae. Gill is a non-conforming design. The size of the articulating surfaces of Gill is relatively small when compared to the overall size of the motion disc.
U.S. Pat. No. 6,039,763 (xe2x80x9cShelokovxe2x80x9d) discloses a motion disc that articulates in a manner resembling the human knee. The disc comprises an articulating concave/convex interface which is bimodal in nature along its lateral plane. In one embodiment, the convex surface features a gradually changing radius of curvature along its anterior-posterior (xe2x80x9cA-Pxe2x80x9d) plane so that A-P flexion provides for A-P translation. In one embodiment, the concave surface includes A-P channels adapted to permit lateralxe2x80x94lateral translation. Lastly, the pair of concave shaped surfaces can be separated by a raised surface to provide a gentle braking of the lateralxe2x80x94lateral translation.
The Shelokov device is non-conforming in both the lateral and A-P planes. There appears to be no means for limiting vertical disengagement of its articulation surfaces. Restraint on pivotal motion of the device is provided by the raised surface between the concave surfaces.
Therefore, there is a need for a motion disc that limits the vertical disengagement of the articulating surfaces.
The present inventors have found that providing a prosthetic motion disc with a locking means advantageously limits the extent of normal disengagement of an articulation interface of the disc during normal use.
The locking means may include any structure located on or in connection with a first prosthetic vertebral endplate that limits the normal disengagement of the articulating surfaces by physically interacting with a structure located on or in connection with the second prosthetic vertebral endplate.
In one embodiment, there is provided a two-piece design comprising first and second prosthetic vertebral endplates, and a locking means holding the two prosthetic vertebral endplates together. In one preferred embodiment, a portion of the locking means extends from one of the articulating surfaces. In another preferred embodiment, the articulation interface is formed in part by an articulation surface formed upon a projection portion of the locking means. Preferably, the articulation interface allows for pivotal motion in any direction, and axial rotation.
Therefore, in accordance with the present invention, there is provided a motion disc comprising:
a) a first prosthetic vertebral endplate having
i) an outer surface adapted to mate with a first vertebral body,
ii) an inner surface comprising a first articulation surface, and
b) a second prosthetic vertebral endplate having
i) an outer surface adapted to mate with a second vertebral body,
ii) an inner surface comprising a second articulation surface, and
c) locking means for limiting the extent of disengagement of the first articulation surface from the second articulation surface.
Preferably, the first and second articulation surfaces are adapted to form an articulating interface.
The present inventors have also found that in preferred embodiments, the device can include a pair of locking surfaces adapted to limit the normal disengagement of the articulating surfaces.
Accordingly, also in accordance with the present invention, there is provided a motion disc comprising:
a) a first prosthetic vertebral endplate having
i) an outer surface adapted to mate with a first vertebral body,
ii) an inner surface comprising a first articulation surface, and
iii) a locking surface, and
b) a second prosthetic vertebral endplate having
i) an outer surface adapted to mate with a second vertebral body,
ii) an inner surface comprising a second articulation surface, and
iii) a locking surface,
wherein the first and second articulation surfaces are adapted to form an articulating interface, and
wherein the first and second locking surfaces are adapted to form a locking interface.
Since, in preferred embodiments, each prosthetic vertebral endplate of the prosthetic device has both an articulating surface and a locking surface, in accordance with the present invention, there is provided a prosthetic vertebral endplate for use in a motion disc, the prosthetic vertebral endplate comprising:
i) an outer surface adapted to mate with a vertebral body, and
ii) an inner surface comprising an articulation surface, and
iii) a locking surface.
In many embodiments, the locking surface is located on the inner surface of the prosthetic vertebral endplate. In many embodiments, the locking surface and the articulation surface face substantially opposite directions.